Many TOP trainers use a 30 blade, particularly at the National level. Note this generally only applies to OPEN HALTER classes.
One reason is that (especially at Nationals) the hair takes longer to grow back, so it saves re-clipping. Also, the closer the blade, the less ''tracks'' you will notice. All colors of horse can be clipped with a 30 blade, and it helps make the definition of a well-muscled fit horse stand out and you can show off every vein and muscle (this has no bearing on a fat our out of shape or underweight horse). It helps make the sleekest line possible on your horse.
That said, I always body clip with a 10 blade, but the few times I have body clipped with a 15 blade I have liked the results better. Just have a lot of 10s laying around so that's what I normally do. And at the local level, my horses stand tied outside the trailer all day and I don't want them to burn.
Andrea